Color postcard from circa 1925-1935 of a bird's-eye view of the Soo Locks, on the St. Marys River, in Sault Sainte Marie, Michigan. The Soo Locks, located on St. Marys River between the United States and Canada, provide ships safe passage around the rapids in the canal. The first locks in this canal were built in 1855, and have been improved upon ever since. The locks are the busiest, based on tonnage passing through, in the world.